Abdominal Aortic Aneurysm Screening Ultrasound

Abdominal aortic aneurysm (AAA) screening ultrasound is a focused, rapid examination of the abdominal aorta from the diaphragm to the bifurcation, using a 2 to 5 MHz curvilinear probe, in the supine position after fasting for 4 to 6 hours (to reduce bowel gas).

The aorta is imaged in the transverse and longitudinal planes. The maximum anteroposterior diameter is measured from the outer wall to outer wall (or inner to inner as the local method requires), perpendicular to the vessel axis, at the widest point, at end diastole or consistently at peak systole.

An aneurysm is defined as an aortic diameter of 3.0 cm or more (or a 1.5-fold local dilatation). The iliac arteries are examined to the extent they are seen. The examination takes 5 to 10 minutes, with no radiation.

Results are classified as normal (under 3.0 cm), small aneurysm (3.0 to 4.4 cm), medium aneurysm (4.5 to 5.4 cm) and large aneurysm (5.5 cm or more).

AAA is a leading cause of sudden death in older men: the prevalence in men aged 65 to 74 years is about 1 to 6% (declining in countries with reduced smoking) and in women about 0.5 to 1%.

Rupture has a mortality of 80 to 90% overall (about 50% of patients die before reaching hospital, with 30 to 50% operative mortality), compared with an elective repair mortality of 1 to 5%.

Annual rupture risk is 0.5 to 1% for 4.0 to 5.0 cm aneurysms, 3 to 15% for 5.5 to 6.9 cm, and 10 to 20% or more for those over 7.0 cm. Screening by a single ultrasound at age 65 in men reduces AAA-related mortality by about 40 to 50% in randomised studies.

Ultrasound has a sensitivity of 95 to 100% and a specificity of 98 to 100% for detecting an AAA in expert hands.

Indications.

• Population screening of men aged 65 to 74 years (single scan; many programmes invite men in their 65th year).
• Targeted screening of women over 65 with risk factors (smoking, family history, cardiovascular disease), and men over 65 with risk factors.
• First degree relatives of patients with AAA (from age 50 to 55 years, men and women).

Patients with peripheral artery disease, popliteal, femoral or thoracic aortic aneurysm, or a connective tissue disorder. Symptomatic patients with abdominal or back pain, pulsatile mass, or unexplained hypotension (an urgent scan). Surveillance of known small aneurysms (3.0 to 3.9 cm every 12 months; 4.0 to 4.4 cm every 6 months; 4.5 to 5.4 cm every 3 months).

Post-repair surveillance (endovascular repair: graft, sac size, endoleak, usually with CT or contrast enhanced ultrasound). Not as screening in young patients without risk factors.

Contraindications and precautions. None. Fasting for 4 to 6 hours reduces bowel gas. Pressure on a tender or suspected symptomatic aneurysm is light.

A symptomatic patient with suspected rupture should not have a delay for imaging; resuscitation and urgent surgical assessment take priority, with bedside ultrasound only to confirm the diagnosis. The person being screened is told about the meaning of the results (a normal result means no further screening; an aneurysm needs surveillance or referral).

A trained operator with documented competence is needed, with quality assurance and measurement quality control (inter-observer variation of 2 to 3 mm).

Preparation and technique. The patient lies supine, with the legs slightly flexed.

The probe is placed in the epigastrium. The aorta (anterior to the vertebral column, to the left of the inferior vena cava) is identified in the transverse plane. It is followed from the diaphragm (at the level of the coeliac axis and superior mesenteric artery) down to the bifurcation at the level of the umbilicus.

Gentle graded pressure displaces bowel gas. Measurements are taken at several levels in the transverse plane (and confirmed longitudinally), using callipers placed on the outer to outer wall (or the inner to inner wall as the screening programme requires.

The same method for follow up), perpendicular to the long axis of the aorta. The maximum diameter is recorded, together with the presence of thrombus, calcification, the proximal extent in relation to the renal arteries (juxtarenal or suprarenal extension) and the iliac arteries.

The images are stored. If the aorta cannot be seen, the study is repeated after more fasting or a different window, or CT is arranged.

Normal appearances. The normal abdominal aorta tapers distally, with a diameter of about 2.0 cm at the level of the diaphragm and 1.5 to 2.0 cm at the bifurcation. It is under 2.0 cm in men and under 1.8 cm in women in most adults; under 3.0 cm is normal by the screening criterion.

It is round on the transverse view, pulsatile, with a thin, smooth, echogenic wall, clear anechoic lumen with no thrombus, and flows with a triphasic or biphasic waveform in the infrarenal aorta. The inferior vena cava lies to the right and is compressible. Aortic calcification may be seen in older people without aneurysm.

Interpretation.

• Normal aorta: diameter under 3.0 cm; no further screening needed.
• Small aneurysm: 3.0 to 4.4 cm; surveillance by ultrasound (3.0 to 3.9 cm every 12 months; 4.0 to 4.4 cm every 6 months, or as the local programme requires), with risk factor management (stop smoking, blood pressure control, statin, antiplatelet therapy).
• Medium aneurysm: 4.5 to 5.4 cm; surveillance every 3 months (or every 3 to 6 months), and vascular review; growth over 1 cm in a year or 0.5 cm in 6 months triggers referral.
• Large aneurysm: 5.5 cm or more in men (5.0 cm or more in women in many programmes) or rapidly expanding: referral to a vascular service within 2 weeks for elective repair (endovascular or open) after CT angiography.
• Symptomatic aneurysm (pain, tenderness) of any size: urgent vascular assessment.
• Morphology: fusiform (uniform dilatation, commonest), saccular (focal outpouching, higher rupture risk at small diameter), mural thrombus (echogenic or hypoechoic crescent along the wall, lumen smaller than outer diameter).
• Other features: calcified wall, inflammatory aneurysm (thick wall, perianeurysmal fibrosis), mycotic aneurysm (irregular saccular with surrounding gas or fluid), and dissection (intimal flap).
• Extent: infrarenal (neck length over 1 to 1.5 cm distal to the renal arteries is suitable for endovascular repair), juxtarenal and suprarenal, and iliac artery involvement (common iliac aneurysm over 1.8 to 2.0 cm).
• Rupture signs: retroperitoneal haematoma (hypoechoic or echogenic collection around the aorta, with loss of the normal wall outline), free intraperitoneal fluid, and discontinuity of the wall; ultrasound can miss a contained rupture.
• Post-repair (endovascular): sac size (a growing sac signals endoleak), graft lumen patency, and endoleak (flow in the sac on colour Doppler or contrast enhanced ultrasound); open repair: graft anastomosis pseudoaneurysm.
• Associated findings: iliac, femoral and popliteal aneurysms, renal artery disease, and incidental abdominal findings (gallstones, renal cysts, hydronephrosis, liver or kidney lesions), which may need follow up.
• Ectatic aorta: diameter 2.5 to 2.9 cm (a risk marker, which may be rescanned in 5 years in some programmes).

Critical values and findings needing immediate action.

• Suspected ruptured AAA (pain, hypotension, pulsatile mass): immediate surgical or endovascular transfer; permissive hypotension (systolic blood pressure about 70 to 90 mmHg while maintaining consciousness), avoid excessive fluids, activate massive transfusion, and avoid delay; imaging only when it does not delay surgery.
• Symptomatic AAA (pain, tenderness) without shock: urgent CT angiography and vascular surgery review within hours.
• Large AAA (5.5 cm or more in men, 5.0 cm or more in women) or rapid expansion: urgent referral to vascular surgery (within 2 weeks).
• Aortic dissection (flap): emergency CT angiography and specialist care.
• Signs of infected (mycotic) aneurysm (fever, high inflammatory markers, gas or fluid around the aorta): urgent antibiotics and surgery.
• Iliac or other peripheral aneurysm with thrombosis or embolisation: vascular review.
• Failure to visualise the aorta in a symptomatic patient: urgent CT.

Limitations and artefacts.

• The aorta is not visualised in 1 to 3% (bowel gas, obesity, previous surgery); a second attempt after fasting or CT is used; a technically inadequate study should be recorded and not reported as normal.
• Measurement variability of 2 to 5 mm between observers and between methods (outer to outer, inner to inner, leading edge to leading edge) affects classification at the 3.0 cm threshold and decisions at the 5.5 cm threshold.
• Ultrasound underestimates diameter compared with CT by 1 to 4 mm on average.
• The same method and operator should be used for surveillance; an oblique cut through a tortuous aorta overestimates the diameter.
• Thrombus and calcification can hide the true outer wall, and the lumen alone should not be measured.
• The suprarenal and thoracic aorta are not reliably seen; CT is needed for planning repair, since ultrasound does not give the neck anatomy, the visceral and iliac access, or the presence of an inflammatory process.
• Ultrasound cannot exclude a contained rupture or an acute aortic syndrome in a symptomatic patient.
• Women have smaller aortic diameters, so the 3.0 cm threshold may miss relative dilatation; and the best threshold for repair in women is lower.
• The yield of screening declines in populations with a low prevalence of smoking; risk based screening is better; incidental findings cause anxiety and follow up costs.
• Screening should not be offered to patients unfit for any repair.
• A normal scan at age 65 provides near lifetime reassurance for men, but new aneurysms can develop later in those with strong risk factors.

Special groups.

• Men aged 65 to 74 years, especially ever smokers (the highest benefit group): single screening scan.
• Women: a lower prevalence (under 1%), with a higher rupture risk at a smaller size; targeted screening in smokers over 65 and with a family history.
• Family history: first degree relatives of people with AAA have a 2 to 4 times higher risk; screen from age 50 to 55.
• Patients with connective tissue disease (Marfan syndrome, Ehlers-Danlos syndrome type IV, Loeys-Dietz syndrome), bicuspid aortic valve, and vasculitis (Takayasu, giant cell arteritis): earlier screening of the thoracic and abdominal aorta.
• Patients with other aneurysms (popliteal, femoral, thoracic): 30 to 50% have an AAA.
• Patients with hypertension, coronary or peripheral arterial disease: higher risk.
• Older patients with comorbidity: the decision for repair is individualised.
• Regions: AAA is less well documented, but syphilitic, tuberculous and mycotic aneurysms, Takayasu arteritis and Behçet disease cause aortic aneurysms in younger people; screening programmes are rare, ruptured aneurysms are often fatal due to delay, and bedside ultrasound is the main diagnostic tool in emergencies.

Follow up tests.

• CT angiography of the aorta (arterial phase) for planning repair, with assessment of neck, access and associated aneurysms.
• Contrast enhanced ultrasound or CT angiography after endovascular repair (first at 30 days, then at 12 months, then yearly or by ultrasound if no endoleak).
• Duplex of the iliac, femoral and popliteal arteries for associated aneurysms.
• Blood pressure measurement and treatment.
• Lipid profile with statin therapy.
• Smoking cessation advice and antiplatelet therapy.
• Fitness assessment (cardiac, pulmonary and renal function) before repair.
• Electrocardiogram and echocardiography when indicated.
• Renal function tests (creatinine, estimated glomerular filtration rate).
• Inflammatory markers and blood cultures for suspected infected aneurysm.
• Referral to vascular surgery.